refactor: move shared GetRectangle to Action base class

Both ActionSelect and ActionZoomWindow had identical 29-line
GetRectangle methods; consolidate into the common base class.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest/Actions/ActionZoomWindow.cs

@@ -166,36 +166,7 @@ namespace OpenNest.Actions
             plateView.Refresh();
         }
 
-        private RectangleF GetRectangle()
-        {
-            var rect = new RectangleF();
-            var pt1 = plateView.PointWorldToGraph(Point1);
-            var pt2 = plateView.PointWorldToGraph(Point2);
-
-            if (pt1.X < pt2.X)
-            {
-                rect.X = pt1.X;
-                rect.Width = pt2.X - pt1.X;
-            }
-            else
-            {
-                rect.X = pt2.X;
-                rect.Width = pt1.X - pt2.X;
-            }
-
-            if (pt1.Y < pt2.Y)
-            {
-                rect.Y = pt1.Y;
-                rect.Height = pt2.Y - pt1.Y;
-            }
-            else
-            {
-                rect.Y = pt2.Y;
-                rect.Height = pt1.Y - pt2.Y;
-            }
-
-            return rect;
-        }
+        private RectangleF GetRectangle() => GetRectangle(Point1, Point2);
 
         public enum Status
         {